1. Field of the Invention
This invention relates generally to the apparatus and method to form conductive coatings over cables for shielding. More particularly, this invention relates to new apparatus and method for form a thin conductive layer over the surface of a signal transmission cable with uniform and precisely controlled configuration for shielding the cable from electromagnetic interference (EMI) such that high performance cables for high frequency signal transmission may be provided and the cables can be manufactured in a continuous process to achieve a low production cost
2. Description of the Prior Art
Current technology of cable manufacture and the layer structures of cables produced from applying the state of the art manufacture processes impose an intrinsic limitation to the usefulness of coaxial cable for transmitting signals with bandwidth at higher frequency. Specifically, this limitation is caused by the use of a thin conductive foil, e.g., an aluminum foil over a polyester or mylar backing, as a shielding layer in current technology for cable manufacture. For broadband signal transmission, system engineers are confronted with problems of signal egress and ingress caused by electromagnetic interference (EMI). In order to minimize the EMI effect, a critical design consideration for coaxial cable manufacture is to provide effective cable shielding. A layer of aluminum foil covering a coaxial cable is commonly employed as the shielding layer. The foils is often glued on to prevent movement in broad-band transmission cables. As the cable is bent or twisted, the foils which is glued on are often peeled off or cracked when subject to shear and stress forces. The protection provided by these conductive foils against EMI interference is weakened. As a result, current technology for cable manufacture, particularly the processes employed to produce the shielding layer for the coaxial cables, cannot satisfy the quality requirement necessary for carrying out high level of performance for broadband high frequency signal transmissions. For this reason, optical fibers are commonly employed to replace the coaxial cables for broadband high frequency signal transmissions due to the difficulties in cable manufacture technology that a high quality shielding layer for the coaxial cables suitable for broadband transmission cannot be effectively produced.
A key parameter used to measure the quality of cable for signal transmission is commonly known as structural return loss (SRL). This SRL generally represents the reflection and attenuation of signal transmission due to imperfections of a cable constructions such as imperfections in uniformity, layer structure and shielding configuration of the coaxial cable. A major difficulty confronted by those involved in cable manufacture is the signal loss, i.e., the SRL, caused by a poor quality of the shielding layer due to current limitations in cable manufacture technology. Specifically, current technology, by employing a wrapping foil structure, still has great deal of difficulties to produce a high quality uniform shielding layer. Referring to FIGS. 1A to 1B for the layer structure of a conventional coaxial cable 10. The coaxial cable 10 includes a conductor wire 15 composed of cooper or steel covered by cooper. A multiple layer structure wraps around the conductor wire 15 which includes a dielectric layer 20 composed of dielectric materials such as foam polyethylene. A shielding layer 25 which includes laminated tape of foil/film bonded to the dielectric layer 20 with a layer of adhesive. A metal braid layer 30 covered by an outer cable jacket 40 are formed to provide protection and overall cable tensile strength. For broadband transmission starting about 50 MHz, virtually all the shielding effectiveness is provided by the foil shield 25. However, the seam of the foil is a source of EMI leakage due to manufacture defects or foil damages caused by cable bending or twisting. The braid layer 30 is simply to provide to the connectors, normally crimped on devices, a holding on structure. FIG. 1B shows a wrapping around configuration of the shielding layer 25 consisting of the foil/film layer structure. In order to assure complete coverage of the entire surface for maximum shielding protection, the conductive foil, e.g., aluminum foil, is bonded to the film to prevent foil push back. A shorting fold configuration is employed where the shielding layer is arranged to have an overlapping bottom layer and a top layer. For the purpose of 100% coverage, the bottom layer is bend backward with the aluminum foil facing upward. The top layer then overlaps the bottom layer with the aluminum foil facing down to contact the aluminum layer of the bottom layer to form a shorting fold.
The shorting fold configuration as that shown in FIG. 1B is then wrapped around under a braid layer which is protected by an outer jacket to assure total shield and protection and the layer structure as shown has been commonly employed in the industry for coaxial cables. Such shielding configuration however generates several difficulties for applying coaxial cables for high frequency signal transmission. As discussed above, since this shielding layer is becoming critically important for high frequency signal transmission, the layer uniformity and configuration must satisfy high precision requirements. However, the structure of the shielding layer is intrinsically non-uniform. Also, the shape and exact configuration of the shielding layer 25 are difficult to control during the manufacture process. The mechanical manufacture processes of wrapping around thin foils may often cause uneven shapes and twisted foil structure thus leads to increased amount of layer imperfections and more SRL loss for signal transmission. The overlapping "shorting fold" configuration is not suitable for high frequency transmission due to the overlapping and sharp angle foil interface under the overlapping portion of the foils. Again, when the cable is bent or twisted, foil deformation or damages may become a critical problem. Abnormal signal interference and attenuation in these folding portion or the damaged segments, e.g., a cracked or peel off segments, of the shielding layer are likely to reduce the performance and effectiveness of signal transmission in this types of cables. For these reasons, optical fibers are generally selected for signal transmission when the signal frequency is above certain level.
Therefore, a need still exists in the art of surface coating to provide new apparatuses and methods with new technique, layer structure and materials for providing a shielding layer for the coaxial cables to overcome these limitations and difficulties. It is required that the structural integrity of the dielectric layer for supporting the shielding layer thereon must be maintained without being adversely affected by the method applied to form shielding layer. It is further desirable that the new techniques can provide uniform shielding structure with flexibility to endure cable bending and twisting. Additionally, it is desirable that the manufacture processes can be precisely controllable by applying automated manufacture processes such that high quality coaxial cables suitable for high frequency signal transmission can be mass produced at low costs.